The invention relates to a method and device for dedusting filters for dust-laden waste gases, including several vertically arranged filter elements each having an upper, open end and a lower, closed end, wherein the waste gases flow through the filter elements from outside and the cleaned waste gases are discharged through the open ends of the same, and wherein for dedusting compressed-air blasts are blown into the open ends of the filter elements to be dedusted.
Filters for dust-laden waste gases are used where impurities such as as chips, shreds, fibers or dusts are to be separated from a conveying air and disposed of or recycled. Examples of industrial branches where lots of dust occur include the wood-processing industry, fiber industry, paper industry or petroleum industry. Impurities are, for instance, sucked off the location of a processing machine, and the polluted waste gases, which are referred to as crude gases, are supplied to the filter. A filter, as a rule, is comprised of several filter elements, which may be designed as filter hoses, filter cartridges, filter bags or even filter plates. The crude gas to be cleaned flows through the filter medium from outside towards inside, whereby the impurities and, in particular, dust and aerosol particles are separated on the outer side of the filter element. The purified gas stream leaves the filter element through the upper, open end into a so-called clean-gas chamber and through appropriate ducts reaches consecutively arranged plant components or escapes into the atmosphere.
In the course of time, the deposit formed on the outer walls of the filter elements, which is called filter cake, becomes thicker and thicker, for which reason the filter must be periodically dedusted in order to guarantee the continuous operation of the same. To this end, air is blown in through the open end of the filter element in order to separate the impurities adhering to its outer side. In doing so, dedusting is effected either by a continuing scavenging-air flow or by a short and vigorous compressed-air blast. The advantages of the scavenging-air method reside in a lower pressure of the dedusting air flow and the low mechanical stresses thus exerted on the filter material. Yet, it is disadvantageous that large amounts of scavenging air are required to efficiently remove the filter cake. The presence of movable parts within the dedusting mechanism involves further drawbacks. Moreover, the dedusting effect will be particularly insufficient with critical or tacky dusts as well as high hose resistances. The high volume flows required as well as additional measures such as, for instance, the heating of the scavenging air for hot-gas filtration bring about further economic disadvantages.
By contrast, the compressed-air method offers the advantages that no moving parts are required within the dedusting mechanism and that the method functions well with critical dusts and high hose resistances, leading to optimum dedusting results. Further advantages would include low energy demands, low volume flows of compressed air and no preheating of the dedusting air necessary for hot-gas filtration in order to avoid lower deviations of the dew point. The disadvantages involved in compressed-air methods include high pressures prevailing in the pressure reservoirs as well as thus caused high mechanical loads exerted on the filter medium and hence reduced service lives of the filter elements. In addition, the throwing off of dust particles from a filter element is likely to cause suction through the adjacent filter element, for which reason no sustained removal of the filter cake will be achieved. A device for dedusting filter hoses or filter cartridges by blowing compressed air into an injector nozzle is, for instance, described in AT 377 711 B.
In the dedusting of filters of the initially defined kind, it is distinguished between what is called online dedusting, which is effected without any shutdown of the plant, and so-called offline dedusting, during which a short-term interruption of the circulation is effected. In online dedusting air is blown into individual filter elements, or compressed-air blasts are fed to the latter, during the operation of the filter plant, while the operation of the remaining filter elements is kept going.
Since part of the energy is lost through the upper, open end of the filter element during the dedusting of a filter element such as, e.g., filter hose, methods enabling higher dedusting rates have been developed by at least partially covering during dedusting the open ends of the filter elements to be dedusted, so that the filter element to be dedusted will be more or less free of pressure. Such a device for the successive dedusting of hose filters is, for instance, described in AT 407 840 B.
Another method and device for cleaning a dust separator is described in AT 405 615 B, wherein, in order to ensure a good cleaning effect, the filter elements to be dedusted are blocked on the clean-gas side and swept around by a directed crude-gas flow during dedusting such that the throwing-off of the filter cake from the filter element to be dedusted will be assisted by said crude-gas flow. That method involves the drawback that during dedusting a certain extent of polluted crude gas will sweep around the filter element to be dedusted. Consequently, the applied pressure of the compressed air used for dedusting must be raised, or a poorer dedusting effect will have to be taken into account. The crude-gas flow also causes the filter cake thrown off the dedusted filter element to be conveyed to other filter elements, where it is again sucked in rather than caused to drop into the dust collection funnel by the shortest way possible.
A further disadvantage of known dedusting methods consists in that pressure fluctuations will occur in the filter, or in the overall plant, due to the switching on and off of the filter elements before and after dedusting. Heavy pressure deviations that may affect the whole system are particularly caused during the switching on of a dedusted filter element, or a filter chamber comprised of several filter elements just dedusted. The control of the fan or the like, which is responsible for the discharging of the clean gas, can thus, for instance, fall out of step by sudden pressure changes. Due to the inertia of the fan, pressure deviations cannot be immediately compensated for and hence may lead to vibrations. On the other hand, changes in the pressure difference between the filter plant and its surroundings may also occur on the site of origin of the dust-laden waste gases, for instance in a processing machine or mill or the like, whereby an air flow in the opposite direction of the suction plant may even be caused.
DE 23 45 722 A1 describes a gas filter in which a dedusting nozzle extends over the entire length of the mouths of the filter elements and is attached to a movably arranged nozzle carriage. That arrangement does not comprise several filter modules arranged in a common housing and does not disclose any measures to minimize pressure deviations during the dedusting of filter modules.
DE 27 09 204 A1 describes a method for cleaning flowing gases and a corresponding device in which compressed air is blown into the filter elements in at least two separate pulses, the first pulse serving to loosen the filter cake and the subsequent pulse serving to blast the same off the filter hose.
DE 27 25 438 A1 discloses a method and device for the blow-back proof compressed-air blast cleaning of filter elements, in which an elastically designed membrane valve is arranged on the mouth of the filter body. By hermetically sealing off the open filter hose during dedusting, it is ensured that the compressed air will become fully effective during dedusting, yet pressure deviations in the filter are not minimized.
Finally, DE 28 31 167 A1 shows a filter comprising filter elements arranged within a chamber, wherein during the dedusting of a chamber the latter is locked both on the crude-gas side and on the clean-gas side, thus enhancing the dedusting intensity. The closure of both the crude-gas-side flap and the clean-gas-side flap calls for the use of an overflow valve during dedusting in order to relieve the overpressure prevailing in the closed chamber. A high pressure within the housing would reduce the dedusting effect of a compressed-air blast or of the scavenging air. That arrangement is characterized by high construction expenditures.